1. Field of the Invention
This invention relates to gas motion induced by axial acoustic waves in a ramjet engine and specifically to apparatus for selectively attenuating the upstream motion so induced.
2. Description of the Prior Art
Various modes of acoustic instability can occur in a ramjet engine. The acoustic waves associated with this instability derive their energy through interactions which occur between moving gases in the engine and processes ongoing in the combustion zone within the engine. A portion of the energy released by the combustion process is known to couple with and excite one or more of the acoustic wave modes of the ramjet combustor. Although mild instabilities resulting from such interaction are not critical and may in fact improve combustion efficiency by increasing fuel-oxidizer mixing, strong oscillations, which are capable of causing physical damage due to vibration or adversely affecting engine performance, are highly undesirable. Further, in a supersonic ramjet, axial mode instabilities, if sufficiently strong can create a back pressure within the engine capable of pushing the inlet shock wave out of the engines' inlet nozzle. This condition, in which combustion ceases to occur, is known as "unstart" and is similar to a "flameout" in a turbojet. However, unlike a turbojet, a ramjet is extremely difficult to restart.
Two major classes of accoustic instability have been identified in ramjets. The first class of instability is denominated transverse instability and involves wave motion normal to the axis of symmetry of the engine. The second class of instability is axial or longitudinal instability which involves wave motion parallel to the engines' axis.
Two categories of acoustic suppression devices are in wide use. The primary use of these devices, in aircraft engine applications, is in the attenuation of engine noise as it leaves the engine discharge nozzle and not in the attenuation of acoustic wave induced gas motion internal to the engine. Tuned or narrow band devices are designed to operate at a particular resonant frequency. Broadband devices, such as labyrinths, are designed to operate over a wide range of frequencies. The devices, in general, are effective in reducing transverse acoustic wave amplitudes. Attentuators for reducing axial wave amplitudes within an engine must be designed with specificity taking into account limitations imposed by engine design and performance considerations. An extremely critical factor in the design of an axial mode attenuator is its affect on the fuel injection pattern within the engine.
There had not been a uniformly acceptable solution to the problem of attenuating upstream gas motion induced by axial acoustic waves in a ramjet engine until the apparatus of the present invention was conceived and adapted for use.